Such appendages include a hydrodynamically-active portion that may, under certain circumstances, be associated with various accessories, such as lift-providing foils that extend from the sides of the appendage, or indeed such as streamlined ballast attached to the end of the appendage, as for a keel fin.
Such appendages are traditionally made of steel. They need to combine great structural strength, in particular in lateral bending, while also supporting ballast that present considerable weight, at least for keel fins. Nevertheless, and in particular for keel fins, it is important to reduce the weight of the appendage as much as possible so that the center of gravity of the keel is as low as possible.
More recently, proposals have been made to make such appendages out of composite material, in particular comprising carbon fibers embedded in resin. Nevertheless, that type of fabrication raises various problems, in particular concerning the watertightness of the appendage and how it is to be connected to the hull.